1. Field of the Invention
The present invention relates to a method of examining the purity of dendrimers, especially for examining whether a dendrimer to be tested is pure or impure according to a scaling relation established by a relation of light scattering intensity per concentration (I/c) proportional to molecular weight (M.W.) based on the Debye equation.
2. Description of Related Art
Dendrimers typically represent a spherical three-dimensional or “tree-like” morphology, comprising a core, repeat branched units emanating from the core, and functional groups on the surface. Due to their characteristic of low viscosity, high solubility, and non-crystalline, dendrimers are widely used. For instance, Taiwan patent No. I314565, issued on 11st Sep. 2009, has disclosed a method for producing a flawless and pure dendrimer which can be used as various high-performance materials in fields of chemistry, medicine, and electronics industry. Moreover, Taiwan patent No. I385193, Taiwan patent No. I319767, and U.S. Pat. No. 7,205,428, have disclosed applications of dendrimers in macromolecular materials and semiconductor materials, and indicated preference for using dendrimers with high purity (e.g. Taiwan patent No. I385193, revealing that luminescence properties of LED may be affected by purity of dendrimers). Although purity of dendrimers is known to play an important role in the use of materials, there is still in lack of an effective method for examining the purity of dendrimers.
Some state-of-the-arts analytical technologies have also been employed to characterize the dendrimers, including matrix assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) and nuclear magnetic resonance (NMR) spectroscopy. However, these instrument are expensive and with many limitations. The MALDI-MS is only a semi-quantitative method in nature due to the inhomogeneous depositions of assayed compounds on the sample ionization plate. Moreover, to acquire the most accurate spectra, various matrixes are usually needed to survey. In addition, the dendrimer ionization efficiencies are sometimes too low to obtain reproducible mass spectrum. NMR spectrum of organic molecules provides reliable structural information, which ensures that the impurity can be clearly observed when mixture sample is assayed. Nevertheless, the low resolution of NMR spectra of macromolecules leads to the difficulties of signal identification, especially for large size of macromolecules, and the situation gets worse for dendrimers while tracing the progress of preparation.
In an early paper (J. Phys. Chem., 1947, 51, pp 18-32), relation of the light scattering intensity of non-absorbed light passing through solution versus solute concentration has been derived. Regardless the solute homogeneity, the following mathematical formula, known as Debye scattering equation, is applicable to determine the molecular weight of a polymer.H(c/I)=1/M.W.+2B c where the c, I, and M.W. represent the solute concentration, the intensity of scattering light, and polymer molecular weight, respectively, and H and B are two constants. In a dilute solution, where the second order term of concentration is negligible, the ratio of scattering light intensity to solute concentration I/c is directly proportional to polymer molecular weight (M.W.). When H is characterized using a standard polymer solution, the measured ratio of light scattering intensity to concentration of a polymer solution, I/c is able to determine the molecular weight of polymer samples. However, the dendrimer structures are of a rigid sphere instead of the configuration of long chain polymers such as random coil or rod-like shape. Hence conventional polymer standards, e.g. polyethylene oxide or dextran, are not applicable to calibrate static light scattering (LS) detector to directly determine the molecular weight (M.W.) of dendrimers to ensure the purity of synthetic compounds, especially high generation products. In such a case, the determination of dendrimer molecular weight (M.W.) to examine the purity of dendrimers remains challenging.